Method of epitaxial germanium tin alloy surface preparation

ABSTRACT

Methods of preparing a clean surface of germanium tin or silicon germanium tin layers for subsequent deposition are provided. An overlayer of Ge, doped Ge, another GeSn or SiGeSn layer, a doped GeSn or SiGeSn layer, an insulator, or a metal can be deposited on a prepared GeSn or SiGeSn layer by positioning a substrate with an exposed germanium tin or silicon germanium tin layer in a processing chamber, heating the processing chamber and flowing a halide gas into the processing chamber to etch the surface of the substrate using either thermal or plasma assisted etching followed by depositing an overlayer on the substantially oxide free and contaminant free surface. Methods can also include the placement and etching of a sacrificial layer, a thermal clean using rapid thermal annealing, or a process in a plasma of nitrogen trifluoride and ammonia gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Nonprovisional patentapplication Ser. No. 13/456,500 (APPM/16989US), filed Apr. 26, 2012,which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Technology described herein relates to the surface preparation ofgermanium tin (GeSn) or silicon germanium tin (SiGeSn) layers forsubsequent deposition.

2. Description of the Related Art

Germanium was one of the first materials used for semiconductorapplications such as CMOS transistors. Due to vast abundance of siliconcompared to germanium, however, silicon has been the overwhelmingsemiconductor material of choice for CMOS manufacture. As devicegeometries decline according to Moore's Law, the size of transistorcomponents poses challenges to engineers working to make devices thatare smaller, faster, use less power, and generate less heat. Forexample, as the size of a transistor declines, the channel region of thetransistor becomes smaller, and the electronic properties of the channelbecome less viable, with more resistivity and higher threshold voltages.

Carrier mobility is increased in the silicon channel area by usingsilicon-germanium stressors embedded in the source/drain areas, whichenhances the intrinsic mobility of silicon. For future nodes, however,still higher mobility devices are needed.

Switching to higher mobility materials than silicon, such as germaniumfor pMOSFETs, has been suggested. However, the mobility of germanium isnot superior to strained silicon, unless the germanium is also strained.It has been recently discovered that germanium tin (GeSn) grown on thesource drain region has the requisite strain for making a superiorgermanium pMOSFET channel, which takes advantage of the germanium/GeSnlattice mismatch. GeSn and silicon germanium tin (SiGeSn) also havemobilities still higher than Ge so they can potentially be used inchannel applications by themselves.

However, during the formation and subsequent treatment of the GeSnlayer, the surface can become oxidized or affected by other impurities,affecting the subsequent deposition of any overlayer. The overlayermaterials can include Ge, doped Ge, a GeSn layer, a SiGeSn layer, adoped GeSn layer, a doped SiGeSn layer, an insulator, or a metal. Unlikesilicon surfaces, germanium surfaces are not effectively passivated byoxide formation. The formation of unstable germanium oxides underatmospheric conditions lead to point defects in the surface which canlead to defects in subsequently deposited layers. Thus, there is a needfor methods of preparing the surface of GeSn or SiGeSn for subsequentoverlayer deposition.

SUMMARY OF THE INVENTION

Methods for preparing the surface of germanium tin (GeSn) or silicongermanium tin (SiGeSn) layers for subsequent deposition are provided. Inone or more embodiments, a layer of Ge, doped Ge, a GeSn layer, a SiGeSnlayer, a doped GeSn layer, a doped SiGeSn layer, an insulator, or ametal can be deposited on a prepared GeSn or SiGeSn layer by positioninga substrate with an exposed GeSn or SiGeSn layer in a processingchamber, heating the processing chamber to a first temperature, flowingan etching gas, such as a halide gas, into the processing chamber at thefirst temperature, etching the surface of the substrate at the firsttemperature using either thermal etching or plasma assisted etching,depositing an overlayer on top of the cleaned surface, where theoverlayer can include Ge, doped Ge, a GeSn layer, a SiGeSn layer, adoped GeSn layer, a doped SiGeSn layer, an insulator, or a metal.

In one or more embodiments, a method of preparing the surface ofgermanium tin (GeSn) or silicon germanium tin (SiGeSn) layers caninclude depositing a sacrificial protective cap germanium layer, whichcan be from 20 Å to 40 Å thick, on the surface of the GeSn or SiGeSnlayer, where the germanium layer is deposited after GeSn or SiGeSn layerformation and before the wafer transfers out of the processing chamber,transferring the wafer to a second processing chamber in which furtherdeposition is to be performed, heating the second processing chamber toa first temperature, flowing a halide gas into the second processingchamber at the first temperature, etching the surface of the substrateat the first temperature using either thermal etching or plasma assistedetching to remove the sacrificial germanium layer and expose the cleanGeSn or SiGeSn surface, prior to depositing another layer on top of thecleaned surface, where the overlayer can include Ge, doped Ge, a GeSnlayer, a SiGeSn layer, a doped GeSn layer, a doped SiGeSn layer, aninsulator, or a metal.

In another embodiment, a method of preparing the surface of germaniumtin (GeSn) or silicon germanium tin (SiGeSn) layers can includepositioning a substrate with an exposed GeSn or SiGeSn layer in aprocessing chamber, flowing H₂ into the chamber while maintaining aconstant pressure, heating the chamber to a first temperature whilemaintaining the flow of H₂, wherein the temperature can be higher than450° C. and can be maintained for a short period of time, stopping theflow of H₂ into the chamber, cooling the chamber to a second temperaturewhich can be below 400° C., prior to depositing another layer on top ofthe cleaned GeSn or SiGeSn surface, where the overlayer can include Ge,doped Ge, a GeSn layer, a SiGeSn layer, a doped GeSn layer, a dopedSiGeSn layer, an insulator, or a metal.

In a further embodiment, a method of preparing a clean GeSn or SiGeSnsurface can include positioning a substrate in a processing chamber,wherein the substrate includes an exposed GeSn or SiGeSn layer,adjusting the processing chamber to a first temperature, flowing an NF₃and NH₃ plasma gas mixture into the processing chamber at the firsttemperature to form a salt mixture, wherein the salt mixture includesthe surface contaminants, heating the substrate to a second temperatureto sublimate the salt mixture, positioning the cleaned substrate in adeposition chamber wherein the vacuum is maintained, and depositing anoverlayer on top of the cleaned surface, wherein the overlayer comprisesone of either Ge, doped Ge, a GeSn layer, a SiGeSn layer, a doped GeSnlayer, a doped SiGeSn layer, an insulator, or a metal.

The halide gas can include chlorine or hydrogen chloride. The dopant caninclude one or more of composed of one of either boron (B), phosphorus(P), or arsenic (As).

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a flow diagram summarizing a method according to oneembodiment.

FIG. 2 is a flow diagram summarizing a method according to anotherembodiment.

FIG. 3 is a flow diagram summarizing a method according to a furtherembodiment.

FIG. 4 is a flow diagram summarizing a method according to a furtherembodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

Methods for preparing the surface of germanium tin (GeSn) or silicongermanium tin (SiGeSn) layers for subsequent deposition are provided.The methods are described in more detail with reference to the figuresbelow.

FIG. 1 is a flow diagram summarizing a method 100 according to oneembodiment. A semiconductor substrate is positioned in a processingchamber at step 102. The semiconductor substrate may be anysemiconductive material on which a stressor layer or an overlayer of anyother function is to be formed. A silicon or germanium substrate onwhich a GeSn or SiGeSn layer is formed may be used in one example. Thesemiconductor substrate may have regions of oxide or surfacecontaminants formed on the GeSn or SiGeSn layer, which may be created attransfer steps between a vacuum environment and an ambient environment.For example, oxide may be formed and surface contaminants may beaccumulated during the transfer steps between formation of asource/drain region and removal of the masking prior to furtherdeposition.

The processing chamber is heated to a first temperature at step 104. Thefirst temperature can be from 300° C. to 400° C. which is chosen toprepare the GeSn or SiGeSn layer for subsequent steps. The temperaturechoice is dependent on a number of factors including the thickness ofthe layer and the amount of time that the layer will be at the chosentemperature.

The main problem expected with GeSn or SiGeSn layers and increasedtemperature is Sn segregation from the GeSn or SiGeSn lattice. Agermanium crystal usually has a cubic structure with unit cell dimensionabout 566 pm. Each germanium atom has a radius of about 125 pm, whichtin atoms have radius of about 145 pm. Adding the larger metal atoms toa germanium crystal matrix results in a larger lattice size that exertsa uniaxial compressive stress to lateral germanium atoms and/or biaxialtensile strain to overlying germanium atoms. Such strain increases theenergy of local electrons and reduces the bandgap of the germanium,resulting in higher carrier mobility compared to unstrained germanium.

In one aspect, the silicon or germanium substrate may have a germaniumchannel layer adjacent to which the stressor layer is to be formed aspart of a transistor gate structure. The GeSn or SiGeSn stressor in thiscase applies a uniaxial stress onto the neighboring germanium layer. Inanother aspect, the germanium channel layer is deposited over thestressor layer, so that a biaxial tensile strain is applied to thegermanium channel layer. However, when the temperature is either toohigh or is held at a high temperature for too long, the tin canpartially or completely segregated from the germanium crystal matrixreducing the stress benefits on the bandgap. As such, time spent at aparticular temperature should be considered for all steps.

An etching gas is flowed into the processing chamber at the firsttemperature at step 106. The etching gas can be a halide gas. The halidegas can be a gas such as chlorine or hydrogen chloride. However, anyhalide gas is contemplated for the etching process. The choice oftemperature above may also be affected by the choice of halide gas. Anexemplary temperature for short etches to remove the surface oxide layeror contaminants with chlorine can be as low as 300° C. When usinghydrogen chloride in one embodiment, etching temperatures can be as lowas from 350° C. to 370° C. The etching gas can be flowed into thechamber at rates such as from about 10 sccm to about 300 sccm, such asfrom about 50 sccm to about 200 sccm, for example about 100 sccm. Theetching gas may also be mixed with a carrier gas to achieve a desiredspace velocity and/or mixing performance in the processing chamber.

Once the etching gas is present in the chamber, the surface oxide orcontaminants can be cleaned from the surface of the GeSn or SiGeSn layerat step 108. The etching process can be done using either a purelythermal etching or plasma assisted etching process. When using theplasma assisted process, etching temperatures can be lower than purelythermal etching processes. As can be expected, since the plasma can beperformed at a lower temperature, the process can be done for a longerperiod of time before Sn segregation occurs.

An overlayer is deposited on the surface of a cleaned GeSn or SiGeSnlayer at step 110. The overlayer can be composed of Ge, doped Ge, a GeSnlayer, a SiGeSn layer, a doped GeSn layer, a doped SiGeSn layer, aninsulator, or a metal. Insulators usable as overlayers includes a listof all known insulators usable in semiconductor applications, withexemplary embodiments including germanium oxide or silicon oxide. Metalsusable in overlayers include a list of all known metals usable insemiconductor applications, with exemplary embodiments including nickeland platinum.

The overlayer can be deposited by any known technique in the art todeposit the described layers, such as chemical vapor deposition (CVD) orphysical vapor deposition (PVD). In one or more embodiments, germaniumoxide and silicon oxide can be deposited by a CVD process. In furtherembodiments, nickel or platinum can be deposited to form contacts by aPVD process. In one or more CVD processes, the precursors may be mixedwith an inert gas, which may be a non-reactive gas such as nitrogen gas,hydrogen gas, or a noble gas such as helium or argon, or a combinationthereof. The dopant that can be incorporated in the layer can includephosphorus, boron or arsenic.

In one embodiment, the overlayer can be a doped germanium layer whichcan be deposited by conventional CVD processes. The germanium precursorcan be a germanium hydride, such as germane (GeH₄), or higher hydrides(Ge_(x)H_(2x+2)), or a combination thereof. The germanium precursor canbe delivered with a dopant source gas. The dopant source gas can includediborane, phosphine, arsine or combinations thereof. Further, the dopantcan be may be mixed with an inert gas, which may be a non-reactive gassuch as nitrogen gas, hydrogen gas, or a noble gas such as helium orargon, or a combination thereof to achieve a desired space velocityand/or mixing performance in the processing chamber.

FIG. 2 is a flow diagram summarizing a method 200 according to anotherembodiment. A semiconductor substrate is positioned in a firstprocessing chamber at step 202. As previously discussed, thesemiconductor substrate may be any semiconductive material on which astressor layer is to be formed. A silicon or germanium substrate onwhich a GeSn or SiGeSn layer is formed may be used in one example.

A GeSn or SiGeSn layer may be formed on the surface of the substrate atstep 204. The GeSn or SiGeSn layer may be formed by any known method inthe art, such as MOCVD. At this point, the first processing chambershould remain sealed to prevent contamination of the GeSn or SiGeSnlayer.

A sacrificial protective cap germanium layer can be deposited on thesurface of the substrate at step 206. The germanium precursor istypically a germanium hydride, such as germane (GeH₄) digermane (Ge₂H₆),or higher hydrides (Ge_(x)H_(2x+2)), or a combination thereof. Thegermanium precursor may be mixed with an inert gas, which may be anon-reactive gas such as nitrogen gas, hydrogen gas, or a noble gas suchas helium or argon, or a combination thereof. The ratio of germaniumprecursor volumetric flow rate to carrier gas flow rate may be used tocontrol gas flow velocity through the chamber. The ratio may be anyproportion from about 1% to about 99%, depending on the flow velocitydesired.

The sacrificial protective cap germanium layer in step 206 should bedeposited prior to the processing chamber seal being broken and prior tothe wafer being transferred from the processing chamber. Further, it isdeposited sequentially after the GeSn or SiGeSn layer is depositedwithin the same chamber. It is important that the GeSn or SiGeSn layershould be kept from exposure to oxygen or other possible contaminants,until the germanium layer is deposited over the surface of the GeSn orSiGeSn layer. The sacrificial protective cap germanium layer can act asa barrier allowing transfer of the substrate with the GeSn or SiGeSnlayer between chambers or to allow opening of the current processingchamber without contamination of the surface of the GeSn or SiGeSnlayer.

The sacrificial protective cap germanium layer in this step can bedeposited over all exposed surfaces, such as over exposed silicon,germanium, the GeSn or SiGeSn layer or photoresists. The sacrificialprotective cap germanium layer is used as a protective coating over theGeSn or SiGeSn layer, prior to transfer from the processing chamber. Thesacrificial protective cap germanium layer can be relatively thin. Insome embodiments, the germanium layer can be from 20 Å-100 Å thick, suchas 20 Å-40 Å thick with preferred embodiments of 20 Å thick.

The sacrificial protective cap germanium layer can be deposited by a CVDprocess using precursors listed above. Growth of the germanium layer isgenerally epitaxial for high structural quality. Pressure in theprocessing chamber can be maintained between about 20 Torr and about 200Torr, such as between about 20 Torr and about 80 Torr, for example about40 Torr. Temperature is between about 150° C. and about 500° C., such asbetween about 300° C. and about 450° C., for example about 300° C.Pressures may be below about 20 Torr in some embodiments, but reducedpressure also reduces deposition rate. Deposition rate at theseconditions is between about 50 Å/min and about 500 Å/min.

The substrate can be transferred to a second processing chamber in step208. The germanium layer deposited over the GeSn or SiGeSn layer in theprevious step will be oxidized and contaminated during the transfer butit will prevent oxidation or other contamination of the GeSn or SiGeSnlayer.

The second processing chamber is heated to a first temperature at step210. The first temperature can be from 300° C. to 400° C. which ischosen to prepare the GeSn or SiGeSn layer for subsequent steps. Thetemperature choice is dependent on similar factors, such as thicknessand time, as described in more detail above.

An etching gas is flowed into the second processing chamber at the firsttemperature at step 212. The etching gas can be a halide gas. The halidegas can be a gas such as chlorine or hydrogen chloride. However, anyhalide gas is contemplated for the etching process. The choice oftemperature above may also be affected by the choice of halide gas. Aknown temperature for short etches to remove the surface oxide layer orcontaminants with chlorine is as low as 300° C. When using hydrogenchloride in one embodiment, etching temperatures can be as low as from350° C. to 370° C. The etching gas can be flowed into the chamber atrates such as from about 10 sccm to about 300 sccm, such as from about50 sccm to about 200 sccm, for example about 100 sccm. The etching gasmay also be mixed with a carrier gas to achieve a desired space velocityand/or mixing performance in the processing chamber.

Once the etching gas is present in the chamber, the sacrificialprotective cap germanium layer can be etched from the surface of theGeSn or SiGeSn layer at step 214. The etching process can be done usingeither a purely thermal etching or plasma assisted etching process. Whenusing the plasma assisted process, etching temperatures can be lowerthan purely thermal etching processes. As can be expected, since theplasma can be performed at a lower temperature, the process can be donefor a longer period of time before tin segregation from GeSn or SiGeSnoccurs. In this embodiment, only the sacrificial protective capgermanium layer was exposed to atmospheric conditions after the previousprocessing steps, which prevents oxide formation or contamination on theGeSn or SiGeSn layer. Thus, by removing the sacrificial protective capgermanium layer, the newly exposed GeSn or SiGeSn layer is substantiallyoxide free and contaminant free, and ready for deposition in subsequentsteps.

An overlayer is deposited on the surface of a cleaned GeSn or SiGeSnlayer at step 216. As stated previously, the overlayer can be composedof Ge, doped Ge, a GeSn layer, a SiGeSn layer, a doped GeSn layer, adoped SiGeSn layer, an insulator, or a metal. The overlayer may bedeposited by any technique known in the art for deposition of suchlayers, such as CVD or PVD. Other parameters and exemplary embodimentsfor overlayer deposition, as described with reference to FIG. 1, areapplicable here as well. For sake of brevity, they are incorporatedherein by reference.

FIG. 3 is a flow diagram summarizing a method 300 according to anotherembodiment. A semiconductor substrate is positioned in a rapid thermalprocessing chamber at step 302. As previously discussed, thesemiconductor substrate may be any semiconductive material on which astressor layer or a layer of any other function is to be formed. Asilicon or germanium substrate on which a GeSn or SiGeSn layer is formedmay be used in one example.

H₂ can be flowed into the processing chamber at step 304. The flow of H₂can be maintained at a constant pressure. In one or more embodiments,the pressure can be maintained from 20 torr to 200 torr.

The chamber can be heated to a first temperature while maintaining theH₂ flow at step 306. The chamber is heated to a temperature greater than400° C. High temperatures can lead to thermal desorption of the surfaceoxide layer or contaminants on the exposed GeSn or SiGeSn surface. Asbefore, consideration must be given to prevent Sn segregation of theGeSn or SiGeSn layer during heating processes. The thermal stability ofthe GeSn or SiGeSn layer is controlled by a number of factors includingthickness of the layer, the final temperature, the time in the presenceof a specific temperature and the atmosphere. One embodiment of the GeSnor SiGeSn layer can be expected to remain stable at 500° C. for 15minutes.

A constant H₂ flow can increase the thermal desorption of the surfaceoxide and/or contaminants in the presence of high temperatures. Withoutintending to be bound by theory, a constant flow of H₂ can enhance theremoval of surface oxides by at least two mechanisms. In one aspect, H₂can react with surface oxides to form volatile species, which can thenbe removed from the chamber. In another aspect, thermal desorption canbe assisted by purging the atmosphere of previously desorbed oxide orcontaminant species, thereby decreasing the partial pressure of oxide orcontaminant species.

In one or more embodiments of this method, thermal processing can beassisted by the use of UV source, such as a flash lamp. Typicalconventional bulbs in an RTP reactor produce a broad wavelength light.The long wavelength of this light is indicative of the low energyproduced. By using a UV source, the oxide can be desorbed from thesurface of the GeSn or SiGeSn layer at a lower temperature or over ashorter time frame than a rapid thermal anneal processing alone.

Once the GeSn or SiGeSn layer is substantially free of oxide andcontaminants, the flow of H₂ into the atmosphere can be stopped at step308. The GeSn or SiGeSn layer is expected to experience some loss afterthermal desorption. Further there is some overlap between techniqueshere, considering that a thin germanium layer, less than 20 Å, could beremoved by the thermal processing at high temperatures, such astemperatures above 450° C.

After the H₂ flow is stopped, the temperature can be cooled below 400°C. at step 310. The temperatures used in rapid thermal anneal processescan be higher than the maximum temperatures listed because they are forsuch a short time period. As such, the temperature should be lowered assoon after the oxide or contaminant species has desorbed as possible toprevent segregation of the tin from the GeSn or SiGeSn crystal lattice.

An overlayer is deposited on the surface of a cleaned GeSn or SiGeSnlayer at step 312. As stated previously, the overlayer can be composedof Ge, doped Ge, a GeSn layer, a SiGeSn layer, a doped GeSn layer, adoped SiGeSn layer, an insulator, or a metal. The overlayer may bedeposited by any technique known in the art for deposition of suchlayers, such as CVD or PVD. Other parameters and exemplary embodimentsfor overlayer deposition, as described with reference to FIG. 1, areapplicable here as well. For sake of brevity, they are incorporatedherein by reference.

FIG. 4 is a flow diagram summarizing a method 400 according to anotherembodiment. A semiconductor substrate is positioned in a processingchamber at step 402. The processing chamber can be an integrated dryclean chamber, such as a SiCoNi chamber available from AppliedMaterials, Inc, of Santa Clara, Calif. As previously discussed, thesemiconductor substrate may be any semiconductive material on which astressor layer or a layer of any other function is to be formed. Asilicon or germanium substrate on which a GeSn or SiGeSn layer is formedmay be used in one example.

The processing chamber is heated to a first temperature at step 404. Thefirst temperature can be below 65° C., with preferred embodiments offrom 20° C. to 60° C.

The ammonia and nitrogen trifluoride gases are then introduced into thechamber to form a plasma gas mixture at the first temperature at step406. The gases should be introduced under vacuum, as vacuum will bebeneficial to remove volatile components created during the cleaningprocess. The amount of each gas introduced into the chamber is variableand may be adjusted to accommodate, for example, the thickness of theoxide layer to be removed, the geometry of the substrate being cleaned,the volume capacity of the plasma, the volume capacity of the processingchamber, as well as the capabilities of the vacuum system coupled to theprocessing chamber. In one or more embodiments, the gases are added toprovide a gas mixture having at least a 1:1 molar ratio of ammonia tonitrogen trifluoride. In another aspect, the molar ratio of the gasmixture is at least about 3:1 molar ratio (ammonia to nitrogentrifluoride). Preferably, the gases are introduced in the chamber at amolar ratio of from 5:1 (ammonia to nitrogen trifluoride) to 30:1.

The gas mixture can then be converted inside the processing chamber to aplasma using a DC or RF power source. Exemplary embodiments includeusing an RF power source to provide an RF power of from about 5 Watts toabout 600 Watts to ignite the gas mixture creating a plasma. Preferableembodiments include using an RF power of less than 100 Watts. The plasmaenergy dissociates the ammonia and nitrogen trifluoride gases intoreactive species that combine to form a highly reactive ammonia fluoride(NH₄F) compound and/or ammonium hydrogen fluoride (NH₄F.HF) in the gasphase. NH₄F and NH₄F.HF are believed to react with the silicon oxide toform ammonium hexafluorosilicate (NH₄)₂SiF₆, SiF₆, NH₃, and H₂O; and thegermanium oxide to form ammonium hexafluorogermanate (NH₄)₂GeF₆, GeF₆,NH₃, and H₂O. The NH₃ and H₂O are gaseous at reaction temperatures andpressures and are removed from the processing chamber leaving a thinfilm of (NH₄)₂SiF₆ remaining on the oxide free surface of the GeSn orSiGeSn layer.

The processing chamber can then be heated to a second temperature atstep 408. The second temperature can be a temperature of greater than80° C., with preferred embodiments of from 80° C. to 150° C. Attemperature higher than 80° C., the thin film of (NH₄)₂SiF₆ or(NH₄)₂GeF₆ can dissociate or sublimate into volatile SiF₄, NH₃ and HFproducts. After the volatile components are made gaseous, they can beremoved from the chamber leaving only the cleaned surface.

The substrate with the cleaned GeSn or SiGeSn surface can be transferredinto a deposition chamber in the presence of vacuum or an inert gas instep 410. The inert gas can be selected from a list of all inert gases,including nitrogen, argon, helium. The environment in the chamber can bemaintained to assure that no further surface contaminants accumulateduring the transfer. The deposition chamber can be any chamber which canbe used to deposit one of the overlayers previously described.

An overlayer is deposited on the surface of a cleaned GeSn or SiGeSnlayer at step 412. As stated previously, the overlayer can be composedof Ge, doped Ge, a GeSn layer, a SiGeSn layer, a doped GeSn layer, adoped SiGeSn layer, an insulator, or a metal. The overlayer may bedeposited by any technique known in the art for deposition of suchlayers, such as CVD or PVD. Other parameters and exemplary embodimentsfor overlayer deposition, as described with reference to FIG. 1, areapplicable here as well. For sake of brevity, they are incorporatedherein by reference.

Methods of preparing a clean surface of GeSn or SiGeSn layers forsubsequent deposition are provided. An overlayer of Ge, doped Ge,another GeSn or SiGeSn layer, a doped GeSn or SiGeSn layer, aninsulator, or a metal can be deposited on a prepared GeSn or SiGeSnlayer by positioning a substrate with an exposed GeSn or SiGeSn layer ina processing chamber, heating the processing chamber and flowing ahalide gas into the processing chamber to etch the surface of thesubstrate using either thermal or plasma assisted etching followed bydepositing an overlayer on the substantially oxide free and contaminantfree surface. In other embodiments, a method of preparing a cleansurface of a GeSn or SiGeSn layer can also include the placement andetching of a sacrificial layer with a subsequent deposition of theoverlayer. In further embodiments, the method of preparing a cleansurface of a GeSn or SiGeSn layer can also include a thermal desorptionusing rapid thermal annealing. In still further embodiments, the methodof preparing a clean surface of a GeSn or SiGeSn layer can also includeflowing an NF₃ and NH₃ plasma gas mixture into the processing chamberform a salt mixture that includes the surface oxide and contaminants,and sublimating the salt. Removing the surface oxides and contaminantsensures the film quality of subsequently deposited layers and thequality of interface between the overlayer and the GeSn or SiGeSn layer.Thereby, this pre-deposition processing leads to lower product loss issubsequent processing.

While the foregoing is directed to embodiments of the invention, otherand further embodiments of the invention may be devised withoutdeparting from the basic scope thereof.

What is claimed is:
 1. A method of preparing a surface, comprising: heating a substrate in a processing chamber to a first temperature, the substrate comprising a layer comprising Ge and Sn; removing contaminants from a surface of the layer comprising Ge and Sn; and depositing a second layer on the surface of the layer comprising Ge and Sn.
 2. The method of claim 1, wherein removing the contaminants comprises using an etching gas.
 3. The method of claim 2, wherein the etching gas is delivered to the layer comprising Ge and Sn at a temperature greater than 300° C.
 4. The method of claim 2, wherein the etching gas comprises a halide gas.
 5. The method of claim 1, wherein the first temperature is from 300° C. to 400° C.
 6. The method of claim 1, wherein the layer comprising Ge and Sn further comprises a dopant.
 7. The method of claim 6, wherein the dopant comprises phosphorus (P), boron (B), or arsenic (As).
 8. The method of claim 1, wherein the second layer comprises Ge, doped Ge, GeSn, SiGeSn, doped GeSn, or doped SiGeSn.
 9. A method of preparing a surface, comprising: heating a substrate in a processing chamber to a first temperature, the substrate comprising a layer comprising Ge and Sn; forming a sacrificial layer on the surface of the layer comprising Ge and Sn; transferring the substrate to a second processing chamber; and removing the sacrificial layer.
 10. The method of claim 9, wherein the sacrificial layer is from 20 Å to 40 Å thick.
 11. The method of claim 9, wherein the sacrificial layer is removed using an etching gas comprising a halide gas.
 12. The method of claim 9, wherein the first temperature is from 300° C. to 400° C.
 13. The method of claim 9, wherein the sacrificial layer is formed at a temperature between about 150° C. and about 500° C.
 14. The method of claim 9, wherein the layer comprising Ge and Sn comprises a dopant.
 15. The method of claim 9, wherein the dopant comprises phosphorus (P), boron (B), or arsenic (As).
 16. A method of preparing a surface, comprising: heating a substrate in a processing chamber to a first temperature, the substrate comprising a layer comprising Ge and Sn; delivering an activated gas comprising ammonia and nitrogen trifluoride gas to the surface of the substrate, the nitrogen-containing gas forming one or more salts on a surface of the layer comprising Ge and Sn; heating the substrate to a second temperature to sublimate the one or more salts, creating a cleaned surface; and depositing a second layer on top of the cleaned surface, wherein the second layer comprises Ge, doped Ge, a GeSn layer, a SiGeSn layer, a doped GeSn layer, a doped SiGeSn layer, an insulator, or a metal.
 17. The method of claim 16, wherein the activated reactive gas is delivered at a temperature from 20° C. to 60 ° C.
 18. The method of claim 16, wherein the one or more salts are sublimated at a temperature from 80° C. to 150 ° C.
 19. The method of claim 16, wherein the activated reactive gas is introduced under vacuum.
 20. The method of claim 16, wherein the ammonia to nitrogen trifluoride are introduced in the processing chamber at a molar ratio of from 5:1 (ammonia to nitrogen trifluoride) to 30:1. 